Thermoplastic film, and in particular polyolefin materials, have been used for some time in connection with packaging of various articles including food products which require protection from the environment, an attractive appearance, and resistance to abuse during the storage and distribution cycle. Suitable optical properties are also desirable in order to provide for inspection of the packaged product after packaging, in the distribution chain, and ultimately at point of sale. Optical properties such as high gloss, high clarity, and low haze characteristics contribute to an aesthetically attractive packaging material and packaged product to enhance the consumer appeal of the product. Various polymeric materials have been used to provide lower gas permeability in order to reduce the transmission of oxygen through the packaging film and thereby retard the spoilage and extend the shelf life of products such as food items which are sensitive to oxygen.
It is also often desirable to include in a packaging film a shrink feature, i.e., the propensity of the film upon exposure to heat to shrink or, if restrained, create shrink tension within the packaging film. This property is imparted to the film by orientation of the film during its manufacture. Typically, the manufactured film is stretched in either a longitudinal (machine) direction, a transverse direction, or both, in varying degrees to impart a certain degree of shrinkability in the film upon subsequent heating. After being so stretched, the film is rapidly cooled to provide this latent shrinkability to the resulting film. One advantage of shrinkable film is the tight, smooth appearance of the wrapped product that results, providing an aesthetic package as well as protecting the packaged product from environmental abuse. Various food and non-food items may be and have been packaged in shrinkable films.
It is sometimes also desirable to orient a packaging film and thereafter heat set the film by bringing the film to a temperature near its orientation temperature. This produces a film with substantially less shrinkability, while retaining much of the advantages of orientation, including improved modulus and optical properties.
Ethylene vinyl alcohol copolymer (EVOH) is known as an oxygen barrier material, and has been used in the past in conjunction with multi-layer packaging films. U.S. Pat. No. 4,064,296 issued to Bornstein et al discloses a film formed by the coextrusion of hydrolyzed ethylene vinyl acetate (HEVA) with outside layers of, for example, ethylene vinyl acetate copolymer (EVA).
Also of interest in U.S. Pat. No. 4,464,443 issued to Farrell et al showing the use of EVOH in a multi-layer polymer structure, and including drying agents or desiccants such as sodium phosphate-di-basic and calcium chloride. EVOH, although a good barrier material, is moisture sensitive, and loses a great deal of its barrier properties at higher levels of relative humidity.
Also of interest is U.S. Pat. No. 4,457,960 issued to Newsome which discloses use of EVOH and EVOH blends in a multiple layer film. The film may be made as shrinkable film, and may be melt extruded.
Also of interest is U.S. Pat. No. 4,495,249 issued to Ohya et al and disclosing a multi-layer film with a core layer of a saponified copolymer of ethylene and vinyl acetate, and including two outer layers of a mixture of copolymer of ethylene and vinyl acetate and a copolymer of propylene and ethylene or linear low density polyethylene. The multi-layer laminate film of the reference can be made heat shrinkable and has gas barrier properties.
Also of interest is U.S. Pat. No. 4,400,428 issued to Rosenthal et al which discloses a composite film having a biaxially oriented polypropylene based film (BOPP) laminated on at least one surface with a multi-layer structure including a gas barrier layer of a hydrolyzed ethylene vinyl acetate copolymer and a layer adjacent to the base film, and a heat sealable outer layer which may be, for example, modified propylene/ethylene copolymer. Adhesion-promoting layers of modified polyolefin may include polypropylenes containing grafted units of alpha, beta-monounsaturated dicarboxylic acids.
U.S. Pat. No. 4,501,797 issued to Super et al discloses an unbalanced oriented multiple layer film including a first layer of polypropylene, a second layer of an anhydride modified polypropylene, and a third layer of ethylene vinyl alcohol.
It is an object of the present invention to provide a coextruded thermoplastic multilayer film characterized by good oxygen barrier properties over a wide range of moisture conditions.
It is a further object of the present invention to provide a thermoplastic multilayer film having an aesthetic appearance with good clarity, and other desirable optical properties.
It is another object of the present invention to provide a thin thermoplastic multilayer film having superior toughness and abrasion resistance.
It is still another object of the present invention to provide a coextruded thermoplastic multilayer film which may be totally coextruded, and oriented to provide a film with good shrink properties and good barrier properties over a wide range of moisture conditions.
It is yet another object of the present invention to provide a coextruded thermoplastic film which is oriented yet substantially shrink-free.